The present invention relates to a method for determining an exposure which is used in the process of photographic printing, and more particularly relates to a method for determining an exposure by which an appropriate exposure or exposure correction can be determined for the purpose of exposing a photosensitive material such as a photographic paper on the basis of image data obtained when an original image formed on a photographic film is scanned.
When an ordinary photograph is taken, it has been experimentally known that the average reflectances of three primary colors including blue (B), green (G) and red (R), which will be referred to as B, G and R hereinafter, are approximately constant. Accordingly, in a conventional photographic printing apparatus, photographic prints of balanced density and color are obtained in this manner: large area transmission density (LATD) with regard to the entire area of an original image is measured; an exposure is determined according to the measured LATD so that an exposure given to each photosensitive layer including B, G and R provided on a photographic paper can be controlled to be constant.
The aforementioned method is disadvantageous in that an appropriate photographic print can not be provided when a luminance distribution or color distribution of a photographic object is biased. An original photographic image, the luminance or color distribution of which is biased, is referred to as a subject failure, when the luminance distribution of a photographic object is biased, the original photographic image is referred to as a density failure, and when the color distribution is biased, the original photographic image is referred to as a color failure. It has been experimentally known that about 65% of original images can be satisfactorily finished in an actual photographing process conducted by the method of LATD.
The following technique disclosed in the official gazette of Japanese Patent Publication No. 2691/1981 is well known for the purpose of automatically adjusting an exposure with regard to a density failure. According to the technique, first, an original image formed on a negative photographic film is scanned; region characteristic values of each image region, that is, the maximum density, minimum density and average density of each image region are found from an image density obtained in the aforementioned scanning operation; the original image is classified according to the obtained characteristic values; and an exposure for the original image is adjusted according to a function of the characteristic value which has been previously determined with regard to each classification. According to the aforementioned technique, the image is divided into several regions in such a manner that the screen of an original image is divided into an upper and a lower portion, a right and a left portion, and a center and a peripheral portion.
Specifically, original images are classified in this manner: the densities of the upper, lower, right, left and central portion are compared with each other, and the original images are classified into two groups, one is a group of original images, the density difference of which is small, and the other is a group of original images, the density difference of which is large. An original image, the density difference of which is small, corresponds to an original image in which open flat scene, the sea or snow, for example, is photographed. An original image, the density difference of which is large and the density of the center is higher than that of the peripheral portion, corresponds to an original image of strobe in dark. In the case of an original image, the contrast of the peripheral portion of which is high, it is classified into lateral camera angle and longitudinal camera angle when the density difference is compared between the upper and the lower portion, and the right and the left portion. When the density difference between the right and left portion is large as compared with that between the upper and lower portion, it can be judged that the picture has been taken in the longitudinal position.
In the case of general photography in which a picture is taken in a normal position, the sky is taken in the upper portion of a screen, and the ground is taken in the lower portion of the screen, and further, illuminating light is supplied from the sky. If the density of upper position in higher than that of lower position in negative film, the low exposure is proper because the outdoor scene is taken under blue sky.
However, there are various photographing conditions which will be described as follows:
The inserting direction of a negative film is not constant in a printing process, so that the film position sometimes becomes upside-down; in some cameras, the direction of film loading is reverse; and even when the same original image is utilized, the inserting direction of a negative film differs according to the size of a photographic printing paper.
For that reason, in some cases, original images can not be classified correctly, and an appropriate exposure or exposure correction can not be provided.
In order to provide an appropriate exposure or exposure correction not depending on the condition of an original image, that is, the image is normal or inverse, the following technique is disclosed in the Japanese Patent Application Open to Public Inspection No. 195439/1989:
First, an original image on a photographic film is scanned. Then, two-dimensional image data is found from the image density obtained through the scanning operation. According to the aforementioned image data, datum of each pixel is sorted and arranged. After that, region characteristic values of each region on the image are found on the basis of the arranged two-dimensional image data. According to the region characteristic values, the original image is classified. Then, the exposure for the original image is adjusted with a function which has been previously determined with regard to each class.
In the aforementioned case, the following procedure is shown as a method for sorting and arranging information of each pixel:
On the basis of the obtained two-dimensional image data, a total or average of the image data is found with regard to the row or line direction. According to the result of the aforementioned calculation, the entire pixel values of the row or line are arranged in the order of increasing power or decreasing power. After the aforementioned sorting and arranging operation has been performed, the two-dimensional image data representing the image structure is lost. However, density data can be uniformly treated irrespective of the condition of the image, that is, the image is normal or inverse, or irrespective of the camera angle. Consequently, an appropriate exposure or exposure correction can be provided in accordance with the condition of the original image.
On the other hand, the following method is disclosed in the official gazettes of Japanese Patent Publication Nos. 52367/1988 and 19533/1991.
In the official gazette of Japanese Patent Publication No. 52367/1988, the following method is disclosed: Consideration is given to the matter that the same scene is continuously photographed in the same printing order in many cases. Similarity between the present frame and the previous frame is judged on the basis of the image characteristic values. The exposure of the frame which has been judged similar, and the temporary exposure of the present frame are averaged so that the final exposure of the present frame can be determined.
In the official gazette of Japanese Patent Publication No. 19533/1991, the following method is disclosed: In order to calculate the exposure of the present frame to be printed, similarity between a plurality of frames and the present frame is found, and the temporary exposure of the present frame is weighted in accordance with the similarity, or the temporary exposure of the present frame is weighted in accordance with the relation between the recording position of the present frame and that of the plurality of frames.
In both cases mentioned above, consideration is given to the continuity of quality of finished prints in the same printing order.
According to the aforementioned exposure determining method disclosed publicly, an exposure or exposure correction is determined as follows:
Characteristic values are calculated from two-dimensional image data obtained when an original image is scanned. According to the obtained characteristic values, the original image is classified into one of a plurality of predetermined sets. An exposure correcting function which has been previously stored correspondingly to each set, is applied to the original image so that the exposure or the exposure correction is determined.
In the aforementioned example, original images are classified as follows:
The original images are classified into a plurality of sets in accordance with the average density of the entire original image. The original images are classified into a plurality of sets in accordance with the contrast between the primary photographic object (the center of the original image) and the background (the peripheral portion of the original image). Results of the aforementioned classifications are combined so that further classification is performed.
According to the aforementioned methods, the threshold values of classification are determined with a statistic method on the basis of the characteristic values of a large number of original images (referred to as a population, hereinafter). For example, in the case where original images are classified into a plurality of sets in accordance with the contrast between a primary photographic object and a background, the density difference between the center and the peripheral portion of each original image is calculated, and a plurality of classification threshold values are determined on the basis of the calculated density difference so that the threshold values can be appropriate with regard to the population.
In general, in one printing order of a negative film, the same scene is photographed in the negative film. Although the same scene is photographed in one negative film, its photographing conditions are slightly different in many cases, for example, the camera angle is different, the size of a primary photographic object is different, the position of the photographic object in the screen is different, and the photographing exposure is different. Consequently, there is a possibility that the region characteristic values obtained from these original images such as the maximum density, minimum density and average density of each divided region, are different. In the case of the aforementioned classification method in which consideration is given to particular region characteristic values (the density difference between the center and the peripheral portion of each original image), and in which the original images are classified into a plurality of sets according to the characteristic values or the results of comparison between the region characteristic values and the reference value, the sets classified depend only upon the characteristic value, so that there is a tendency that the original images are classified into different sets although the scenes on the original images are similar. Accordingly, a different exposure or exposure correction is required to be calculated.
As a further example, in the case in which original images are classified into a plurality of sets in accordance with the average density of the entire original image, there is a high possibility that a plurality of original images in which the same scene has been photographed a plurality of times with slightly different photographic exposures, are classified into different sets. Consequently, it is not possible to continuously and stably obtain an exposure or exposure correction in the same printing order.
To keep up a good standard of photographic printing in laboratories, they attach importance to the stability and continuity of density and color balance of the photographic prints in the same printing order. When photographic prints in the same printing order lack continuity and stability, it is necessary to reprint them, and if this occurs frequently, it lowers the productivity of the laboratory and increases the cost of photographic prints. Therefore, it has been desired to overcome the disadvantages which occur in the aforementioned conventional technique.
According to the aforementioned Japanese Patent Publication Nos. 52367/1988 and 19533/1991, a method is disclosed to calculate a similar exposure or a similar exposure correction in the case of a similar scene.
The aforementioned method is disadvantageous in that: When the similarity is not judged correctly, not only an appropriate exposure can not be calculated for the present frame, but also the exposure of a plurality of frames which have been misjudged to be similar, is affected.
According to the latter method, the continuity of print quality can be a little improved in one printing order. However, in this case, it is necessary to determine a temporary exposure for a series of frames before an exposure for the present frame is determined, and further it is necessary to calculate the similarity of the present frame with regard to other frames. Accordingly, the calculation becomes complicated, and it takes a long period of time, and further the processing capability of the photographic printing apparatus is lowered.
The present invention has been achieved in order to solve the aforementioned problems in the prior art. It is a primary object of the present invention to provide a method for determining an exposure that: specific image characteristic values are taken up, and the values are compared with each other or they are compared with a predetermined reference value; according to the result of the comparison, the fluctuation of an exposure or an exposure correction caused when the original images are classified into a plurality of image sets, is absorbed; and photographic prints of constant quality can be effectively produced without depending on the photographing exposure and the position of a primary photographic object.